1. Field of the Invention
The invention relates to a method for avoiding the formation of water marks, and more particularly to a method for avoiding the formation of water marks during drying of cleaning procedure after well implantation.
2. Description of the Prior Art
The formation of water marks is a serious problem during drying of cleaning procedure, particular after spin-drying. There are several drawbacks of water marks. First, they result in the formation of non-uniform oxide thickness and further the happening of non-uniform dopant distribution after following implantation and thermal cycles. Second, they maybe confuse any succeeding in-line inspections and further disturb the process fluency.
As shown on FIG. 1, a conventional manufacture flow, that water marks frequently happen is a procedure of performing well implantation. First, an insulating layer, such as tetra-ethyl-ortho-silicate (TEOS), is deposited on a silicon substrate by using low pressure chemical vapor deposition (step 110). For the mask alignment, the insulating layer is generally thick enough to meet the requirement. A photoresist layer is formed on the insulating layer. Then a pattern defining a well region in the silicon substrate is transferred into the photoresist layer by lithography method (step 111) to expose partial surface of the insulating layer that the well region is defined thereunder. Next, the exposed insulating layer is removed by conventional buffered-oxide etch (BOE) dip (step 112). After removing the exposed insulating layer, a bare zone of the silicon substrate is formed. The corner between sidewalls of the insulating layer and the bare zone is very shaped because of height variation from topside of the photoresist layer and the bare zone of the silicon substrate.
Implants for the well are then implanted into, the photoresist layer, the insulating layer and the silicon substrate (step 113). Next, a hard skin, resulting from well implantation on the photoresist, is first removed by oxygen plasma ashing (step 114). Finally the residue of the photoresist layer is removed by the conventional wet chemical clean technology, such as using Caro""s acid (sulfuric acid and hydrogen peroxygen) and RCA-1(APM plus HPM, APM=NH4OH+H2O2+H2O; HPM=HCL+H2O2+H2O) (step 115), followed by spin-drying. During the period of spin-drying, a little water drops maybe stop at the corners between the sidewalls of the insulating layer and the bare zone of the silicon substrate because of large height variation. Those water drops definitely result in the formation of water marks.
As depicted in FIG.2A, a silicon substrate 120 is provided and an insulating layer 121 is deposited thereon. The insulating layer 121 is formed by the method of low pressure chemical vapor deposition and has a thickness about 3000 Angstrom so as to be for mask alignment requirement.
Next, a photoresist layer 130 is first formed on the insulating layer 121. The thickness of the photoresist layer 130 is about 9000 Angstrom. Then a pattern for defining well region in the silicon substrate 120 is transferred into the photoresist layer 130 so as to exposure partial surface of the insulating layer, shown on FIG. 2B. Next, the insulating layer 121 exposed by the photoresist layer 130 is first removed to expose the bare zone 150 of the silicon substrate 120 shown on FIG. 2C. The removal of the exposed insulating layer 121 uses a conventional BOE dip. After removing the exposed insulating layer 121, the height variation between topside of the photoresist layer 130 and the bare zone 150 is about 1.2 um. Implants 140 are implanted into the photoresist layer 130, the insulating layer 121 and the silicon substrate 120. Followings are cleaning procedures. First, a hard skin (not shown), resulting from the implantation on the photoresist polymer 130, is removed by using oxygen plasma ashing. Then residue of the photoresist layer 130 is removed by using conventional method, such as the Caro""s acid plus RCA-1 solution. Finally a wafers boat is dried by the conventional method, such as spin-drying. Generally, during spin-drying, water drops, stopping at the corners of the bare zone 150, can""t be spin-out because of large height variation. The addition of the hydrophobic bare zone 150, those water drops form water marks about 0.1 mm in diameter at the corners between the insulating layer and the hydrophobic bare zone of the silicon substrate.
However, In-situ removal of the insulating layer and the photoresist layer by combination of BOE dip and the Caro""s acid without plasma ashing can avoids the formation of water marks. But it is difficult to remove the hard skin, resulting from well implantation, by the BOE dip and the Caro""s acid. Accordingly, the conventional process has some drawbacks for performing well implantation. The main shortcoming is the formation of water marks because of the hydrophobic bare zone of the silicon substrate.
It is an object of the present invention to provide a method for avoiding water marks formed during cleaning step after well implantation. A native oxide is formed on an exposed silicon substrate to avoid the formation of water marks.
It is another object of the present invention to provide a method for forming the hydrophilic surface of the silicon substrate during cleaning step after well implantation. The hydrophilic surface can avoid the formation of water marks.
In the present invention, a method comprises providing a silicon substrate. An insulating layer is deposited on the silicon substrate and a photoresist layer formed thereon. A well pattern is transferred into the photoresist layer to expose partial the insulating layer thereunder the well defined. Implants are implanted into the photoresist layer, the insulating layer and the silicon substrate. Then the insulating layer exposed by the photoresist layer is removed and in-situ a native oxide is formed on the silicon substrate thereunder the well defined, whereby avoids water marks formed at the corner of the insulating layer and the silicon substrate. In-situ removal of the insulating layer and formation of the native oxide are by using BOE dip plus Caro""s acid dip so that the native oxide can change the surface of the silicon substrate from hydrophobic into hydrophilic to avoid the formation of water marks.